A nonaqueous electrolyte battery such as a lithium ion battery is characterized by a high energy density and thus has been widely used as a power source for portable equipment such as a portable telephone and a notebook personal computer. In recent years, the capacity of the nonaqueous electrolyte battery is likely to increase further as the performance of the portable equipment becomes higher. In parallel with this, it is also important to ensure safety.
In the current nonaqueous electrolyte battery, e.g., a polyolefin microporous film with a thickness of about 20 to 30 μm is used as a separator that is interposed between a positive electrode and a negative electrode. However, the commonly used separator easily shrinks when the temperature in the battery becomes extremely high, and can cause a short circuit. Therefore, as a means of improving the safety of the nonaqueous electrolyte battery, the heat resistance of the separator may be improved.
On the other hand, e.g., when the nonaqueous electrolyte battery in a charged state is placed in a high-temperature environment, heat can be generated from the positive electrode to raise the temperature in the battery further. Therefore, as a means of improving the safety of the nonaqueous electrolyte battery, a positive active material having high thermal stability also may be used.
For example, Patent Documents 1 and 2 propose a nonaqueous electrolyte battery that uses a positive active material having high thermal stability along with a separator having good heat resistance. However, in the technology as disclosed in Patent Document 1, there is a limit to the reduction in thickness of the separator because a nonwoven fabric or paper is used for the separator. For example, if the separator has a thickness of 30 μm or less and an active material such as graphite is used for the negative electrode, a short circuit may be likely to occur. Moreover, the technology as disclosed in Patent Document 2 focuses only on thermal shrinkage of the separator at 120° C. Thus, it is difficult to ensure the safety of the battery at temperatures higher than this.
Patent Documents 3 to 7 propose a multilayer separator that includes a high heat-resistant layer as a technology for preventing a short circuit due to thermal shrinkage of the separator. However, the separator needs to be further improved to achieve the safety of the battery at even higher temperatures.
For example, in the case of an electrode body obtained by winding the positive electrode and the negative electrode with the separator interposed between them, a short circuit can occur due to thermal shrinkage of the separator in the width direction. Thus, it is necessary to suppress the thermal shrinkage in the width direction.
Patent Document 1: JP 2004-296325 A
Patent Document 2: JP 2004-303474 A
Patent Document 3: JP 2006-351386 A
Patent Document 4: WO 2007/66768
Patent Document 5: JP 2007-273123 A
Patent Document 6: JP 2007-273443 A
Patent Document 7: JP 2007-280911 A